The invention relates to near-infrared analyzers which operate on the principle of transmittance and interactance (transflectance) and, in particular, to such analyzers which are portable.
Near-infrared analyzers are relatively well-known and are used to analyze such diverse properties as the octane content of gasoline, the moisture content of cheese, and the oil/protein content of grain. They operate by passing near-infrared light into a sample that is to be tested and measuring the intensity of selected frequencies of the light that either passes through the sample or that is reflected back from the sample.
With relatively translucent materials such as gasoline, most of the light is able to pass through the sample. Therefore, relatively little amplification of the signal generated by the photodetector is needed. For such materials, the analyzer may be calibrated before testing of the sample simply by performing an analysis run on the empty test chamber. With other, relatively opaque materials such as grain or other food products, however, much of the light is absorbed or blocked by the material. Accordingly, when testing the material, it is necessary to amplify by greater amounts the photodetector signal to be able to extract the information used to analyze the content of the sample.
For portable or hand-held near-infrared analyzers configured to analyze grain or other relatively opaque samples, it has been customary to provide a sealed calibration standard having known parameters of interest for each type of grain or other material that is to be analyzed. The calibration standard is inserted into the analyzer first and a calibrating analysis run is performed on the standard. After the analyzer has been calibrated, the calibration standard is removed and the sample to be tested is inserted into the analyzer and analyzed.
Because the transmisivity of the calibration standard and the test sample are relatively the sane, the photodetector signal generated when analyzing the test sample is not amplified any more than the photodetector signal generated when analyzing the calibration standard is. In other words, the analyzer operates at a single gain. This has been standard procedure for the past several years because it was believed that a dual gain portable analyzer was impractical. This is because noise in the signalxe2x80x94which tends to be present in the photodetector signal to a far greater extent in a hand-held unit than in a larger, better shielded laboratory or table-top unitxe2x80x94becomes amplified as well, and it had been believed that such noise amplification would make analysis of the signal unreliable.
Using calibration standards to calibrate the analyzer using the calibration standard is not ideal, however. This is because the standards tend to get soiled or smeared with debris as they are handled, and this can taint the calibration or otherwise degrade instrument performance. Additionally, the standards constitute extra equipment that can be lost and which needs to be carried with the analyzer. Accordingly, a portable, hand-held, near-infrared analyzer for analyzing grain or other relatively opaque material that does not need calibrating standards to operatexe2x80x94i.e., one which can calibrate itself using an empty chamberxe2x80x94is desirable.
The present invention bucks the conventional wisdom and provides a near-infrared analyzer having two or more gain values associated with the photodetector signal. This allows the analyzer to be calibrated on an empty test chamber using a first, low gain value, and then the sample to be tested using a high gain value. Gain switching is effected by altering the resistance in the feedback path around the op-amp used to amplify the photodetector signal.